1. Field of the Invention
The present invention relates to an electric potential sensor and an electronic apparatus using the same, and more particularly to an electric potential sensor employing a piezoelectric tuning fork and an electronic apparatus using the same.
2. Description of the Related Art
In an electronic apparatus such as an electrophotographic copying machine or a laser printer, a photosensor drum must be electrified to a desired level. To this end, an electric potential sensor is used to perform non-contact measuring of the surface potential of the photosensor drum.
FIG. 5 is a block diagram of a conventional electric potential sensor. As shown in FIG. 5, an electric potential sensor 1 is provided with a piezoelectric tuning fork 2xe2x80x2, an oscillation circuit 9, pre-amplifier 10, an amplifier 11, a detecting circuit 12, a low-pass filter 13, a DC amplifier 14, and an output terminal 15. The piezoelectric tuning fork 2xe2x80x2 comprises a tuning fork 2 having two legs 3 and 4 (generally made of a metal), a driving piezoelectric member 5 provided on one of the legs 3, and a feedback piezoelectric member 6 provided on the other leg 4. A detecting electrode 7 is provided on the leg 3 opposedly facing a surface 8 to be detected, and a capacitance C1 is formed between the detecting electrode 7 and the surface 8. The surface 8 indicates, for example, the surface of the photosensor drum.
An output from the oscillation circuit 9 is connected to the driving piezoelectric member 5, and the feedback piezoelectric member 6 is connected to an input of the oscillation circuit 9.
The detecting electrode 7 is connected, through the pre-amplifier 10 and the amplifier 11, to the synchronous detecting circuit 12. The output of the feedback piezoelectric member 6 is also connected to the synchronous detecting circuit 12. The output from the synchronous detecting circuit 12 is connected to the output terminal 15 via the low-pass filter 13 and the DC amplifier 14.
FIGS. 6A to 6D show the waveforms of the signal in respective parts of the electric potential sensor 1 described above. An operation of the electric potential sensor 1 is explained with reference to FIGS. 5 and 6.
When the signal generated by the oscillation circuit 9 is applied to the driving piezoelectric member 5 provided on one leg 3 of the tuning fork 2, the driving piezoelectric member 5 mechanically vibrates to cause the piezoelectric tuning fork 2 to vibrate in a reverse-phase mode in which the two legs of the tuning fork 2 vibrate 180xc2x0 out of phasexe2x80x94that is, as the left leg 3 moves to the left, the right leg 4 moves to the right and vice versa. When the piezoelectric tuning fork 2 vibrates, the feedback piezoelectric member 6 on leg 4 vibrates mechanically and outputs an electrical signal indicative of the frequency, phase and magnitude of the vibration of leg 4. The electrical signal is fed back to the oscillation circuit 9 to form a closed loop comprising the oscillation circuit 9, the driving piezoelectric member 5 and the feedback piezoelectric member 6. This feedback circuit causes self-oscillation to occur at a desired frequency.
When the tuning fork 2 vibrates, the distance between the detecting electrode 7 and the surface 8 (and along with it the capacitance C1) changes as a function of the vibration of the tuning fork 2. The signal output from the detecting electrode 7 changes in accordance with both the change in capacitance C1 and the electrical potential of the surface 8 to be detected. Particularly, the frequency of the output signal varies as a function of the vibration of the tuning fork 2 and the amplitude of the output signal varies substantially proportionally with any change in the electrical potential of the surface 8. The signal output from the detecting electrode 7 is amplified by the pre-amplifier 10 and the amplifier 11 and is input into the synchronous detecting circuit 12. FIG. 6A shows the sinusoidal waveform of the signal Vout output from the detecting electrode 7 and input to the detecting circuit 12.
The signal output from the feedback piezoelectric member 6 is also input to the synchronous detecting circuit 12 as a reference signal. The signal input from the amplifier 11 is synchronously detected by cross correlating it with the reference signal. As shown in FIG. 6B, the signal output from the feedback piezoelectric member 6 is a sine wave having the same frequency and phase as the signal output from the detector electrode 7. The difference between these signals is their amplitude (the amplitude of the output of amplifier 11 changing as a function of the amplitude of the change on the surface b). FIG. 6C shows the waveform of the signal output from the detecting circuit 12. The signal output from the detecting circuit 12 is preferably a signal which is formed by full-wave rectification of the cross correlated signals input to the detecting circuit 12.
The DC component in the signal detected by the detecting circuit 12 is extracted by the low-pass filter 13, and the signal is amplified by the DC amplifier 14 to be output from the output terminal 15. This signal, shown in FIG. 6D, is substantially proportional to the potential of the surface 8.
Due to external factors, the piezoelectric tuning fork will sometimes include unwanted vibration components which are not in reverse-phase with each other. A typical vibration component of this type is an in-phase vibration component wherein the two legs of turning fork 2 move in unison (in-phase). FIG. 7A to 7D show the waveforms of respective parts of the piezoelectric tuning fork 2 when a non-reverse-phase vibration components having a frequency different from the vibration in the reverse-phase mode component is added to the tuning fork 2 in a conventional electric potential sensor.
In the electric potential sensor 1, when the piezoelectric tuning fork 2 vibrates with non-reverse-phase components in addition to reverse-phase components, a signal (FIG. 7A) is output from the synchronous detecting electrode 7 and is then input to the detecting circuit 12. This signal has a waveform in which two or more sine waves having different waveforms are combined. The synchronous detecting circuit also receives the reference signal (FIG. 7B) generated by the feedback piezoelectric member 6. This signal also has a waveform in which non-reverse-phase vibrations are added to reverse-phase vibrations. As described above, this signal has the same waveform and phase (but a different magnitude) as the signal output from the detecting electrode 7.
In the detecting circuit 12, a signal from amplifier 11 which has both reverse-phase mode and non-reverse-phase mode components is cross correlated with the reference signal from piezoelectric member 6 having the same phase in which the vibration in the reverse phase mode is added to the in-phase mode. FIG. 7C shows the waveform of the signal output from the detecting circuit 12. This signal is formed by full-wave rectification of the signal input to the detecting circuit 12.
The DC component in the signal detected by the detecting circuit 12 is extracted by the low-pass filter 13, amplified by the DC amplifier 14 and output to the output terminal 15. FIG. 7D shows the waveform of the signal output from the output terminal 15. As described above, a DC voltage having the predetermined value is output from the output terminal 15. By comparing the voltage value of the signal in FIG. 7D with that in FIG. 6D, it can be seen that the signal output from the output terminal 15 is larger for the case in which the non-reverse-phase mode vibration is added to the reverse-phase mode vibration.
As described above, the conventional electric potential sensor 1 has had a problem that the signal output by the sensor becomes unstable when the non-reverse-phase mode component of vibration is applied to the piezoelectric tuning fork 2 by external factors.
It is therefore an object of the present invention is to provide an electric potential sensor that can stabilize the signal to be output even when the non-reverse-phase mode component of vibration is applied to the piezoelectric tuning fork by external factors, and an electronic apparatus using the same.
In order to accomplish the above object, according to the present invention, a detector electrode is provided on one leg of a piezoelectric tuning fork having two legs opposedly facing a surface to be detected, a driving piezoelectric member is provided on at least one leg of the tuning fork, and a feedback piezoelectric member respectively provided on both legs of the tuning fork.
In addition, the electric potential sensor according to the present invention comprises an oscillation circuit for outputting a signal to be input to the driving piezoelectric member, and an adder or a subtractor for adding or subtracting the signal output from the feedback piezoelectric members provided on respective legs of the tuning fork to feedback to the oscillation circuit.
Furthermore, the electric potential sensor according to the present invention comprises a detecting circuit for synchronously detecting the signal output from the detecting electrode by the signal output from the adder or the subtractor.
Furthermore, an electronic apparatus according to the present invention comprises a photosensor drum and an electric potential sensor for measuring the surface potential of the photosensor drum.
By being constituted as described above, the electric potential sensor according to the present invention can stabilize the signal to be output even when a non-reverse-phase mode component of vibration is applied to tuning fork.
Moreover, in the electronic apparatus according to the present invention, the surface potential of the photosensor drum can be kept at a predetermined value even when a vibration in the non-reverse-phase mode is applied to the electronic apparatus.
In the electric potential sensor according to the present invention, an electrode for detecting is provided on one leg of a tuning fork having two legs opposedly facing a surface to be detected, one or more driving piezoelectric members are provided on at least one of the legs, and at least one feedback piezoelectric member is provided on each of the legs. The electric potential sensor also has an oscillation circuit for outputting a signal to be input to the driving piezoelectric members, an adder or a subtractor for adding or subtracting the output signal from the feedback piezoelectric members provided on-opposite legs of the tuning fork to output a signal which is fed back to the oscillation circuit, and a detecting circuit for synchronously detecting the signal output from the detecting electrode to the reference signal output by the adder or the subtractor. Thereby, even if the vibration in the non-reverse-phase mode is added to the piezoelectric tuning fork by external factors, the output signal is stabilized.
Moreover, in the electronic apparatus of the present invention, the photosensor drum and the potential sensor for measuring the surface potential are provided thereon. Thereby, even if the vibration in the non-reverse-phase mode is applied to the apparatus by the external factors, the surface potential of the drum can be held at the predetermined value.
For the purpose of illustrating the invention, there is shown in the drawings several forms which are presently preferred, it being understood, however, that the invention is not limited to the precise arrangements and instrumentalities shown.